Late complications in the treatment of Hodgkin's disease with radiation therapy are increasingly being recognized. Thyroid dysfunction following irradiation is one such complication which has not been widely recognized, probably because of the subtle clinical nature of minor thyroid hormone imbalance. In addition, it has long been thought that the thyroid was relatively resistant to radiation therapy. Two hundred thirty-two patients with Hodgkin's disease followed at the BCRC have been evaluated for post-therapy thyroid dysfunction. Thirty-nine percent of those with mantle radiation therapy had an elevated TSH, but normal T4, but an additional 26% had an elevated TSH plus a depressed T4. Thus, 65% of 184 patients had evidence of thyroid dysfunction following mantle radiation therapy. Since thyroid hormone replacement is simple, inexpensive and usually without major side effects if properly monitored, it is reasonable in light of the foregoing to place all patients with an elevated TSH on thyroid replacement therapy whether or not they are chemically hypothyroid.